Selective catalyst reduction of nitrogen oxides with hydrogen

ABSTRACT

A system and method for reducing nitrous oxides in diesel exhaust with hydrogen and carbon monoxide over a palladium based catalyst. The catalyst comprises a compound represented by the formula: X % Pd—Y % V 2 O 5 /Z, where X is between about 0.1 to about 2.0, Y is between about 0.1 to about 7.0, and Z is an oxide support.

FIELD OF THE INVENTION

The present invention relates to the catalytic reduction of nitrogenoxides.

BACKGROUND OF THE INVENTION

Nitrogen oxides, in particular NO and NO₂ (NO_(x)), resulting fromcombustion processes continue to be a major source of air pollution.They contribute to photochemical smog, acid rain, ozone depletion,ground level ozone, and greenhouse effects. More than 95% of nitrogenoxide emissions are derived from two sources: ˜49% from mobile sources,such as vehicles, and ˜46% from stationary sources, such as powerplants. Many technologies have been developed in an attempt to decreasesuch emissions.

Three-way catalysis is very effective for removing emissions fromgasoline engines, where narrow band oxygen sensors afford closed loopcontrol with an air:fuel ratio of about 14.07. Diesel engines, on theother hand, operate very lean and with a wide-band air:fuel ratio ofabout 14 to about 24. While diesel engines have considerable benefits togasoline engines, due to the nature of diesel fuel and the compressionignition combustion process, diesel engines emit a high quantity ofparticulate matter and nitrogen oxide emissions. Many catalysts usefulfor gasoline engines are not suitable for use in a diesel engine exhauststream as a wider operating temperature window is required.

The current commercially available technology for reducing NO_(x)emissions from stationary sources is selective catalytic reduction(SCR). Ammonia (NH₃) is widely accepted as the reducing agent of choice.Similar SCR technology is also effectively applied to mobile sources,where NH₃ is usually generated by the thermal decomposition of urea.However, there are many commercial and logistical drawbacks, namely: (1)a separate tank and injection system is required, (2) several issuesexist relating to NH₃ slip, (3) the difficulty of handling ureasolutions during cold conditions, and (4) as of yet, no realinfrastructure exists to widely deploy the necessary urea solution.These factors indicate the desirability of the development of an activeNO_(x) reduction catalyst that makes use of other reductants, such ashydrogen. Hydrogen has been shown to be a promising reductant for NO_(x)under lean burn conditions and will most likely be available inautomobiles from fuel processors for fuel cell applications, on-boardreforming of diesel fuel, or the like.

SUMMARY OF THE INVENTION

The present invention provides an emissions treatment system and methodsfor reducing contaminants in diesel exhaust streams containing nitrousoxides (NO_(x)). The system includes a catalytic converter having atleast one inlet, at least one outlet, and an interior workingenvironment adapted to receive and dispel exhaust. A catalyst isdisposed in the interior working environment and comprises a compoundrepresented by the formula: X % Pd—Y % V₂O₅/Z, where X is a weightpercent between about 0.1 to about 2.0, Y is a weight percent betweenabout 0.1 to about 7.0, and Z is a high surface area support material. Areducing agent is provided comprising H₂ and CO at a ratio of H₂:CO fromabout 1:1 to about 3:1. The reducing agent is mixed with diesel engineexhaust and the mixture is injected into the interior workingenvironment of the converter where the catalyst reduces NO_(x), presentin the exhaust.

Further areas of applicability of the present invention will becomeapparent from the detailed description provided hereinafter. It shouldbe understood that the detailed description and specific examples, whileindicating the preferred embodiment of the invention, are intended forpurposes of illustration only and are not intended to limit the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description and the accompanying drawings, wherein:

FIG. 1 illustrates XRD patterns of the catalysts (a) Al₂O₃; (b) 20%TiO₂/Al₂O₃; (c) 5% V₂O₅/20% TiO₂/Al₂O₃; (d) 1% Pd-5% V₂O₅TiO₂/Al₂O₃; (e)1% Pd/20% TiO₂/Al₂O₃; and (f) 1% Pd-5% V₂O₅/Al₂O₃;

FIG. 2 illustrates TPR profiles of 1% Pd-5% V₂O₅/TiO₂/Al₂O₃ (upper) and1% Pd/TiO₂/Al₂O₃ (lower) catalysts;

FIG. 3 illustrates NO conversion as a function of temperature overvarious catalysts. Reaction conditions: 0.1 g catalyst; total flowrate=200 ml/min; [NO]=500 ppm; [O₂]=5%; [H₂]=4000 ppm; He=balance;

FIG. 4 illustrates H₂ conversion as a function of temperature overvarious catalysts. Reaction conditions: 0.1 g catalyst; total flowrate=200 ml/min; [NO]=500 ppm, [O₂]=5%; [H₂]=4000 ppm; He=balance;

FIG. 5 illustrates effect of space velocity on NO, H₂ conversions and N₂selectivity for H₂-SCR over 1% Pd-5% V₂O₅/TiO₂/Al₂O₃ catalyst. •,1.0×10⁵ h⁻¹; ▪, 5.2×10⁵ h⁻¹; O, 1.8×10⁶ h⁻¹. Reaction conditions:[NO]=500 ppm; [H₂]=4000 ppm; [O₂]=5%; He=balance;

FIG. 6 illustrates NO, H₂ conversions and N₂ selectivity as functions oftemperature over various supported Pd based catalysts. Reactionconditions: 0.05 g catalyst; [NO]=500 ppm; [H₂]=4000 ppm; [O₂]=5%;He=balance; total flow rate=500 ml/min;

FIG. 7 illustrates NO conversion as a function of time on stream at 170°C. over 1% Pd-5% V₂O₅/Al₂O₃(•); at 150° C. over 1% Pd-5% V₂O₅/Al₂O₃(▴);and 1% Pd-5% V₂O₅/TiO₂—Al₂O₃(O) catalysts for H₂-SCR. Reactionconditions: 0.05 g catalyst; [NO]=500 ppm; [H₂]=4000 ppm; [O₂]=5%;He=balance; total flow rate=500 ml/min;

FIG. 8 illustrates NO, H₂, CO conversions and N₂ selectivity asfunctions of temperature over 1% Pd-5% V₂O₅/TiO₂—Al₂O₃ catalyst ondifferent feed compositions. H₂ Conversion (solid line), CO conversion(dashed line). Reaction conditions: 0.1 g catalyst; total flow rate=200ml/min; [NO]=500 ppm; [O₂]=5%; [H₂]=2000-4000 ppm; [CO]=0-2000 ppm;He=balance;

FIG. 9 illustrates dependence of NO conversion rate on NO concentrationon 1% Pd-5% V₂O₅/TiO₂/Al₂O₃ catalyst at 200° C. Reaction condition: 5 mgcatalyst; [H₂]=4000 ppm; [CO]=500 ppm; [O₂]=5%; He=balance; total flowrate=500 ml/min;

FIG. 10 illustrates dependence of NO conversion rate on H₂ concentrationfor 1% Pd-5% V₂O₅/TiO₂/Al₂O₃ catalyst at 200° C. Reaction condition: 5mg catalyst; [NO]=500 ppm; [CO]=500 ppm; [O₂]=5%; He=balance; total flowrate=500 ml/min;

FIG. 11 illustrates dependence of NO conversion rate on CO concentrationfor 1% Pd-5% V₂O₅/TiO₂/Al₂O₃ catalyst at 200° C. Reaction condition: 5mg catalyst; [H₂]=4000 ppm; [NO]=500 ppm; [O₂]=5%; He=balance; totalflow rate=500 ml/min;

FIG. 12 illustrates dependence of NO conversion rate on O₂ concentrationfor 1% Pd-5% V₂O₅/TiO₂/Al₂O₃ catalyst at 200° C. Reaction condition: 5mg catalyst; [H₂]=4000 ppm; [NO]=500 ppm; [CO]=500 ppm; He=balance;total flow rate=500 ml/min;

FIG. 13 illustrates effect of H₂O on NO conversion over 1% Pd-5%V₂O₅/TiO₂/Al₂O₃ catalyst at 200° C. Reaction condition: 0.05 g catalyst;[H₂]=4000 ppm; [NO]=500 ppm; [O₂]=5%; [H₂O]=2.3%; He=balance; total flowrate=500 ml/min;

FIG. 14 illustrates effect of SO₂ on NO conversion over 1% Pd-5%V₂O₅/TiO₂/Al₂O₃ catalyst at 200° C. Reaction condition: 0.05 g catalyst;[H₂]=4000 ppm; [NO]=500 ppm; [O₂]=5%; [SO₂]=20 ppm; He=balance; totalflow rate=500 ml/min;

FIG. 15 illustrates FTIR spectra obtained at different temperatures in aflow containing 4000 ppm H₂+500 ppm NO+5% O₂ over 1% Pd/TiO₂—Al₂O₃. Thetemperatures include: a) 150° C.; b) 200° C.; and c) 240° C.;

FIG. 16 illustrates FTIR spectra obtained at different temperatures in aflow containing 4000 ppm H₂+500 ppm NO+5% O₂ over 1% Pd-5%V₂O₅/TiO₂—Al₂O₃. The temperatures include: a) 150° C.; b) 200° C.; andc) 240° C.; and

FIG. 17 illustrates changes in the FTIR spectra of NH₃ and NH₄ ⁺ uponswitching the gas flow from (4000 ppm H₂+500 ppm NO+5% O₂) to (500 ppmNO+5% O₂) over 1% Pd-5% V₂O₅/TiO₂—Al₂O₃ catalyst. The time afterswitching gas flow is as follows: a) 0 min; b) 1.5 min; c) 3 min; and d)10 min.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description of the preferred embodiment(s) is merelyexemplary in nature and is in no way intended to limit the invention,its application, or uses.

The present invention relates to an emissions treatment system forreducing contaminants in diesel exhaust streams containing nitrousoxides (NO_(x)). While the decomposition of NO_(x) to innocuouscomponents is thermodynamically favored with the temperatures andpressures affiliated with diesel exhaust, the reactions are inhibited byhigh activation energies and require a catalyst to facilitate thedecomposition. As is widely known, however, the performance of manycatalysts deteriorates in the presence of oxygen. Various catalysts havebeen used to decompose NO_(x) and include precious metals, metallicoxides, zeolites, and similar materials fixed to a suitable carrier. Thepresent invention is directed to the novel use of selective catalyticreduction of NO_(x) with hydrogen (H₂-SCR) using palladium. In preferredembodiments, the catalyst reduces the NO_(x) in-situ, and not as part ofa NO_(x) adsorbing reaction.

In various embodiments, the present invention provides an emissionssystem and methods of reducing NO_(x) in lean-burn diesel engineexhaust. The system is operable throughout the wide range of normaloperating temperatures of diesel engines, typically between about 125 toabout 650° C., and has an efficiency of greater than about 87%, morepreferably, greater than about 95%. In various embodiments, the systemoperates with the exhaust having a flow rate of between about 200 andabout 700 kg/hr, and a space velocity through the interior workingenvironment of between about 9,000 and about 70,000 hr⁻¹.

The system of the present invention includes a catalytic converterhaving at least one inlet, an outlet, and an interior workingenvironment adapted to receive and dispel an exhaust. A catalyst isdisposed in the interior working environment and comprises a compoundrepresented by the formula: X % Pd—Y % V₂O₅/Z, where X is a weightpercent between about 0.1 to about 2.0, Y is a weight percent betweenabout 0.1 to about 7.0, and Z is a high surface area support material.The catalyst is incorporated into the exhaust system at such a positionthat yields optimum efficiencies with a fast light-off time period.

The support for the catalyst typically comprises a high surface arearefractory metal oxide. Dispersed within the oxide layer is thepalladium metal component. Without being bound by theory, the palladiumpromotes the oxidation and reduction of nitrogen species and is presentin an amount of between about 0:1 to about 2% by weight of the catalyst.The catalyst coating thickness and supporting materials will varyaccording to the targeted reduction of NO_(x). Non-limiting examples ofsuitable oxide support materials include alumina, titania, zirconia;mixtures of alumina with one or more of titania, zirconia, and ceria;ceria coated on alumina; and titania coated on alumina. The metal oxidemay also comprise a mixed oxide, such as silica-alumina, amorphous orcrystalline aluminosilicates, alumina-zirconia, alumina chromia, andalumina ceria. Presently preferred metal oxides include gamma aluminaand titania coated on alumina. In certain embodiments, the supportmaterial comprises a pillared interlayered clay (PILC), such as Ti-PILC.In various embodiments, the catalyst further comprises at least onepromoter. Non-limiting examples of suitable promoters include: Ce, Mn,Zr, La, Gd, Nb, Pr, Nd, Sm, Eu, and combinations thereof. Typically, thepromoter, if any, is present in an amount between about 0.01 and about20% by weight of the catalyst.

In various embodiments, the metal oxide will have a surface area of fromabout 50 to about 300 m²/g or more. In presently preferred embodiments,the oxide support material is present in an amount having a loading perunit volume of between about 0.5 to about 10 g/in³. As should beunderstood to those skilled in the art, the ratio of substrate length todiameter, or frontal surface area to volume, should be optimized basedon the exhaust flow rate and the targeted NO_(x) reduction.

The SCR catalyst of the present invention can be in the form of selfsupporting catalyst particles, for example as a packed bed, or thecatalyst can be supported on a metal or ceramic honeycomb structure, orthe like. In other embodiments, the catalyst composition can be disposedas a washcoat or as a combination of washcoats on a ceramic or metallicsubstrate.

A reducing agent comprising H₂ and CO is provided to assist in thereduction reactions. In presently preferred embodiments, the reducingagent comprises a ratio of H2:CO of from about 1:1 to about 3:1, withthe latter more preferable. In certain embodiments, hydrocarbons can beused in addition to or in place of the CO. The reducing agent is mixedwith diesel engine exhaust and the mixture is injected into the interiorworking environment for the catalyst to reduce the nitrous oxidespresent in the exhaust. In certain embodiments, a mixing element isprovided having a first input coupled to a reducing agent source, asecond input coupled for receipt of diesel engine exhaust, and at leastone output. The diesel engine exhaust mixed with the reducing agent isinjected into the interior working environment of the converter wherethe catalyst reduces nitrous oxides present in the exhaust. In variousembodiments, the reducing agent is in the form a reducing fuel that isproduced on-board a vehicle. In this instance, a fuel reformer isprovided to convert a sufficient amount of diesel fuel, oxygen, andmoisture into H₂ and CO. In other embodiments, H₂ may also be providedfrom other sources, such as from a fuel cell application.

Catalyst Preparation and Activity Measurement

A 20 wt % TiO₂-on-γ-Al₂O₃ support is prepared by the hydrolysis of asolution of Ti[O(CH₂)₃CH₃]₄ in the presence of γ-Al₂O₃ (PSD-350 gradefrom Aluminum Company of America, BET surface area of approximately 350m²/g, 60-100 mesh). The solid sample is dried in air at about 500° C.for about 6 hours. A 5% V₂O₅/20 wt % TiO₂-γ-Al₂O₃ is prepared byimpregnation in 20% TiO₂-γ-Al₂O₃ with an aqueous solution of NH₄VO₃ inoxalic acid. A similar procedure is used to prepare 5% V₂O₅/Al₂O₃ and 5%V₂O₅/TiO₂ (P25, Degussa, BET surface area of 30.6 m²/g). Afterimpregnation, the catalysts are dried at about 120° C. for about 12hours and calcined at about 500° C. in oxygen for about 12 hours todecompose the ammonium salt into the corresponding oxide. Palladium issubsequently impregnated in 5% V₂O₅/20 wt % TiO₂-γ-Al₂O₃ and 20 wt %TiO₂-γ-Al₂O₃ using a Pd(NH₃)₄Cl₂ aqueous solution. The catalyst is driedat about 120° C. for about 12 hours and calcined at about 500° C. forabout 6 hours in oxygen.

The catalytic activity measurements according to the present inventionare carried out in a fixed-bed quartz reactor. A typical reactant gascomposition comprises: 500 ppm NO, 4000 ppm H₂, 0-2000 ppm CO (whenused), 5% O₂, and the balance He. A 100 mg sample is used in each run.The total flow rate is about 200 ml/min (under ambient conditions).Premixed gases (1.01% NO in He, 5.00% H₂ in He, and 1.0% CO in He) arereadily available from Matheson Tri-Gas of Irving, Tex. Water vapor isgenerated by passing He through a heated saturator containing de-ionizedwater. The NO and NO₂ concentrations are continually monitored using achemiluminescent NO/NO_(x) analyzer (for example, Thermo EnvironmentalInstruments, Inc., Model 42C). The products are analyzed using a gaschromatograph (Shimadzu, 8A) with a 13X molecular sieve column for H₂,CO, and N₂ separation and Porapak Q column for N₂O. Ammonia formation ismonitored by FTIR (Fourier Transform Infrared Spectroscopy). Typically,no ammonia is detected by FTIR in the lean burn conditions. Thecatalytic activity is based on the calculated NO_(x) conversion usingthe following formula.

${N\; O_{x}\mspace{14mu}{conversion}} = {\frac{{{{inlet}{NO}}_{x}\mspace{14mu}({ppm})} - {{{outlet}{NO}}_{x}\mspace{14mu}({ppm})}}{{{inlet}{NO}}_{x}\mspace{14mu}({ppm})} \times 100(\%)}$The N₂ selectivity is calculated as follows:

${N_{2}\mspace{14mu}{selectivity}} = {\frac{\left\lbrack N_{2} \right\rbrack}{\left\lbrack N_{2} \right\rbrack + \left\lbrack {N_{2}O} \right\rbrack} \times 100(\%)}$

Since the reactions are carried out at relatively low temperatures, partof the decrease in NO concentration can be attributed to the adsorptionof NO onto the catalysts. Thus, to minimize this occurrence, thefollowing is done at the beginning of each example: the catalyst isfirst purged with reactant gas until the inlet and outlet NOconcentrations are equal (i.e., about 500 ppm). The temperature issubsequently raised to the desired level. At each reaction temperature,the NO conversion and product analysis is performed after allowing thereaction to reach steady state (about 1-2 hours, depending on thereaction).

The nitrogen balance is calculated for each step using the followingequation: inlet [NO]=outlet [NO]+[N₂]+[N₂O]. Steady-state kineticstudies for the NO reduction by H₂ in the presence of CO and O₂ arecarried out for the 1% Pd-5% V₂O₅/TiO₂/Al₂O₃ catalyst making use of afixed-bed, quartz flow reactor, with 5 mg of catalyst used in each run.The NO concentration in an exhaust is simulated by blending differentgaseous reactants. The typical reactant gas composition is as follows:0-5000 ppm H₂, 100-500 ppm NO, 0-500 ppm CO, 1-5% O₂, and the balanceHe. The total flow rate is about 500 ml/min (under ambient conditions).The same instrumentation, as described above, is used throughout.

Catalyst Characterization

Powder X-ray diffraction (XRD) measurements are carried out on thecatalysts of the present invention using a Rigaku Rotaflex D/Max-Csystem with a Cu Kα (λ=0.1543 nm) radiation source. The samples areloaded with a depth of 1 mm. In each H₂-TPR (temperature-programmedreduction) experiment, a 50 mg sample is loaded into a quartz reactorand pretreated with an O₂/He (100 ml/min) flow at about 500° C. forabout 0.5 hours. The sample is cooled to room temperature in an O₂/Heflow. The reduction of the sample is carried out starting at roomtemperature to about 600° C. in a 5.32% H₂/N₂ flow (of about 40 ml/min)with a temperature ramp of about 10° C./min. The consumption of H₂ ismonitored with the use of a thermal conductivity detector. Waterproduced during the reduction is trapped in a 5 A° molecular sievecolumn.

Infrared spectra are recorded on a Nicolet Impact 400 FTIR spectrometerwith a TGS detector. The samples are prepared as a self-supporting waferof 1.3 cm diameter. This is achieved by compressing 15 mg of the sample.The wafer is loaded into the IR cell (BaF₂ windows). The wafers arepre-treated at 573 K in a flow of high purity O₂/He for about ½ hour andstepwise cooled to room temperature. At each temperature step, thebackground spectrum is recorded in flowing O₂/He. This spectrum issubsequently subtracted from the sample spectrum obtained at the sametemperature step. Thus, the IR absorption features that originated fromthe structure vibrations of the catalyst are eliminated from the samplespectra. IR spectra are recorded by accumulating 100 scans at a spectraresolution of 4 cm¹.

The XRD patterns of the catalysts are shown in FIG. 1. Crystalline PdOphases are not detected in all samples, which indicates that Pd ishighly dispersed on the support. Four diffraction peaks with 2θ=25.3,37.5, 39.4 and 48 respectively, are observed in the titania containingsamples. These peaks originate from the anatase form of titania.

H₂-TPR profiles of 1% Pd-5% V₂O₅/TiO₂—Al₂O₃ and 1% Pd/TiO₂—Al₂O₃catalysts prepared are shown in FIG. 2. All samples show one main peak,which can be assigned to the reduction of Pd (II) to Pd (0). For theV₂O₅ containing sample, the reduction peak temperature is 46° C. higherthan that on V₂O₅-free sample. While not being bound by theory, it isbelieved that the presence of V₂O₅ retards the reduction of Pd oxides.

NO_(x) Reduction on Pd Based Catalysts by H₂ in the Presence of Oxygen

The performance of a range of Pd-based catalysts is shown in FIG. 3. Amaximum NO conversion of 98% is achieved at about 150° C. over a 1%Pd-5% V₂O₅/TiO₂/Al₂O₃ catalyst. This reaction displays only one NOconversion peak. Under the same conditions, a 1% Pd/TiO₂/Al₂O₃ catalystalso shows a high NO conversion. However, two separate NO conversionpeaks are observed at 150° C. and 240° C., respectively. While a maximumNO conversion of 80% is obtained over the 5% V₂O₅/TiO₂/Al₂O₃ catalyst,the peak conversion temperature is the same as the second NO conversionpeak for the 1% Pd/TiO₂/Al₂O₃ catalyst. This may indicate that thesecond NO conversion peak may not depend on the Pd concentration. FromFIG. 3, it can be seen that the addition of V₂O₅ to the 1% Pd/TiO₂/Al₂O₃catalyst increases the NO conversion, especially at temperatures ofaround 200° C. Thus, the addition of V₂O₅ widens the high NO conversiontemperature window to 140-250° C. (with about 80% NO conversion). The 1%Pd/TiO₂/Al₂O₃ catalyst exhibits a narrower NO conversion temperaturewindow of 140° C.-180° C. Based on the investigated temperature range,the NO conversion order is as follows: 1% Pd-5% V₂O₅TiO₂/Al₂O₃>1%Pd/TiO₂/Al₂O₃>5% V₂O₅/TiO₂/Al₂O₃.

FIG. 4 illustrates hydrogen conversions as a function of temperature onvarious TiO₂/Al₂O₃ supported catalysts. The H₂ conversion for eachcatalyst reaches 100% at the temperatures where the maximum NOconversion is observed. On the 1% Pd/TiO₂/Al₂O₃ catalyst, this isachieved at a temperature of 150° C. While for the 1% Pd-5%V₂O₅/TiO₂/Al₂O₃ catalyst, 98% H₂ conversion is reached at 150° C. The 5%V₂O₅/TiO₂/Al₂O₃ catalyst, on the other hand, reaches complete H₂conversion only at 240° C.

For the presently preferred catalyst, 1% Pd-5% V₂O₅/TiO₂/Al₂O₃, theNO—H₂—O₂ reaction activities including NO conversion, H₂ conversion, andN₂ selectivity, at various space velocities are shown in FIG. 5.Generally, the space velocity significantly influences the lowtemperature NO conversion, not the NO conversion in the high temperaturerange. As the space velocity is increased (1.0×10⁵ to 1.8×10⁶ h⁻¹), theNO conversion decreases, and NO conversion maximum shifts toward ahigher temperature (about 150° C.-200° C.). H₂ conversion exhibits asimilar trend, namely as the space velocity increases, H₂ conversiondecreases. The corresponding 100% H₂ conversion temperature alsoincreases. No significant space velocity effects are visible on the N₂selectivity at temperatures below 200° C. A significant decrease in N₂selectivity is observed in the temperature range 200° C.-300° C.

FIG. 6 shows the conversions of NO and H₂, as well as the N₂ selectivityof various supported Pd catalysts as a function of temperature. Asshown, at low temperatures (i.e., less than about 170° C.), the 1% Pd-5%V₂O₅/TiO₂ catalyst has a very low activity, while 1% Pd-5%V₂O₅/TiO₂/Al₂O₃ catalyst and 1% Pd-5% V₂O₅/Al₂O₃ catalyst displayrelatively high activities under the same conditions.

In the high temperature range (i.e., greater than about 240° C.), all ofthe samples show similar activities. The order of increasing activity ofthese Pd based catalysts is: 1% Pd-5% V₂O₅/TiO₂/Al₂O₃>1% Pd-5%V₂O₅/Al₂O₃>1% Pd-5% V₂O₅/TiO₂. The same trend is observed for H₂conversion. The N₂ selectivity of different catalysts is also shown inFIG. 6; all samples show a similar N₂ selectivity.

FIG. 7 illustrates the NO conversion as a function of reaction time forcatalysts 1% Pd-5% V₂O₅/Al₂O₃ (at 150° C. and 170° C.) and 1% Pd-5%V₂O₅/TiO₂—Al₂O₃ (at 150° C.). At 150° C., the 1% Pd-5% V₂O₅/TiO₂—Al₂O₃catalyst has a high NO conversion rate to reach steady state in only 1hour. A similar trend is observed for the 1% Pd-5% V₂O₅/Al₂O₃ catalystat 170° C. At 150° C., however, the 1% Pd-5% V₂O₅/Al₂O₃ catalystdisplays a slow NO conversion rate and only reaches 60% NO conversionafter three hours. The 1% Pd-5% V₂O₅/TiO₂—Al₂O₃ catalyst has a higherconversion rate, indicating that modification of Al₂O₃ with titania hasa positive effect on the conversion rates of the catalyst.

NO_(x) Reduction on Pd Based Catalysts by H₂ and CO in the Presence ofOxygen

In various embodiments, the present invention contemplates an emissionssystem using CO in the H₂-SCR process. FIG. 8 illustrates the influenceof CO on the H₂-SCR reaction over the 1% Pd-5% V₂O₅/TiO₂—Al₂O₃ catalystwith the total reductant concentration at about 4000 ppm. This aspect ofthe present invention surprisingly indicates that the presence of COdecreases the NO conversion, especially in the low temperature range.This result differs from previous theories that predicted enhancement byCO on Pd/TiO₂/Al₂O₃. As the CO concentration is increased, thetemperature of maximum NO conversion shifts from 150° C. to 200° C. Thepresence of CO inhibits the H₂ oxidation reaction as the temperature toreach complete oxidation of H₂ increases from 170° C. to 200° C. as COis added to the feed gas. The effect of CO on N₂ selectivity is verycomplex in the low temperature range, while at high temperatures the N₂selectivity appears to be independent of the presence of CO.

Kinetics Studies for H₂-SCR in the Presence of CO and Excess O₂ in aDifferential Reactor

To determine the reaction order with respect to NO, the concentrationsof H₂, CO, and O₂ are kept constant. The concentration of NO issubsequently varied from 100 ppm to 500 ppm. Similarly, to determine thereaction order with respect to H₂, the concentration of NO and CO arekept constant, while varying the concentration of H₂ between 1000 ppmand 5000 ppm. The flow rate is about 500 ml/min and only 5 mg ofcatalyst is used, with less than 20% NO conversion obtained at 200° C.Thus, the reactor can be treated as a differential reactor. Experimentalresults of the rate of NO conversion as a function of NO, CO, H₂, and O₂concentrations are presented in FIG. 9-12. FIG. 9 shows that the rate ofNO conversion increases linearly as a function of NO concentration. Thereaction rate of NO conversion as a function of reactant concentrationscan be expressed in Equation 1 as follows:r _(NO) =−k _(a)[NO]^(x)[H₂]^(y)[CO]^(z)[O₂]^(m)  (1)where r_(NO) is the SCR rate, k_(a) is the apparent rate constant, andx, y, z, and m are reaction order for NO, H₂, CO, and O₂, respectively.According to FIG. 9, the reaction order (x) with respect to NO is 0.92,thus making this reaction close to first-order.

The rate of NO conversion as a function of H₂ concentration is shown inFIG. 10, and is found to increase with increasing H₂ concentrations. Thereaction order (y) with respect to the H₂ concentration is calculated tobe 0.6. FIG. 11 shows the rate of NO conversion as a function of COconcentration. The rate of NO conversion decreases with COconcentration. Thus, CO inhibits the reaction, which can be attributedto the competitive adsorption between NO and H₂. The reaction order (z)with respect to CO concentration is calculated to be −0.18. The rate ofNO conversion as a function of O₂ concentration is shown in FIG. 12 (500ppm NO, 4000 ppm H₂, 500 ppm CO). When the O₂ concentration increasesfrom about 2% to 10%, the NO consumption rate slightly decreases. Thereaction order (m) with respect to O₂ is calculated to be −0.04.

According to the above results, the H₂-SCR reaction in the presence ofCO and excess O₂ can be considered to be approximately first-order withrespect to NO, 0.6-order to H₂, −0.18-order to CO and −0.04-order to O₂.The rate of NO conversion can be expressed in Equation 2 as follows:r _(NO) =−k _(a)[NO][H₂]^(0.6)[CO]^(−0.18)[O₂]^(−0.04)  (2)

Effects of Water and SO₂ on the H₂-SCR Reaction

Water vapor is a major component in diesel engine exhaust and oftenleads to catalyst deactivation. Resistance of the NO_(x) abatementcatalyst to deactivation by water vapor is an important factor. FIG. 13shows the effect of H₂O on the SCR activity of the Pd—V₂O₅/TiO₂—Al₂O₃catalyst. It should be noted that before the addition of water, the SCRreaction is allowed to stabilize for one hour at 200° C.

The addition of 2.3% H₂O affects only a barely detectable decrease inthe NO conversion. Upon removal of the water vapor, the activity israpidly restored to its original level. The effect of SO₂ on the SCRactivity is another important factor in the H₂-SCR reaction due to thepresence, although very small, of sulfur in the diesel fuel. FIG. 14indicates the effect of SO₂ on SCR activity of the Pd—V₂O₅/TiO₂—Al₂O₃catalyst. Results indicate that a 20 ppm SO₂ concentration at 200° C.rapidly decreases the NO conversion rate within the first 2 hours of thereaction, and the reaction slowly stabilizes to yield a 46% conversion(down from 85%) in about 4 hours. Removal of the SO₂ feed restored theactivity to about 90%.

FTIR Studies

FIG. 15 shows the FTIR spectra obtained at 150° C., 200° C., and 240° C.(4000 ppm H₂+500 ppm NO+5% O₂) over the 1% Pd/TiO₂—Al₂O₃ catalyst. At150° C., two broad peaks are present at 1600 cm⁻¹ and 1300 cm⁻¹. Thevarious bands between 1600-1550 cm⁻¹ and 1300 cm⁻¹ observed may beassigned to the asymmetric and symmetric stretching modes of variouslycoordinated nitrates. The bands at 1904 cm⁻¹ and 1848 cm⁻¹ areattributed to gas phase or weakly adsorbed NO. The band at 1610 cm⁻¹ canbe assigned to adsorbed NO₂. The band at 1270 cm⁻¹ is attributed to thedeformation modes of adsorbed NH₃. Another band at 1620 cm⁻¹ due to thedeformation modes is barely detected because of overlapping by the bandfor nitrate. A very weak band at 1740 cm⁻¹ may be assigned to Pd⁰—NO.The intensity of all the bands associated with nitrates and nitritedecreases as the temperature is increased from 150 to 240° C. while theband at 1270 cm⁻¹ due to adsorbed NH₃ intensifies and dominates at hightemperatures.

The corresponding FTIR spectra obtained over 1% Pd-5% V₂O₅/TiO₂—Al₂O₃are shown in FIG. 16. At 150° C. (spectrum of FIG. 16), the bands due toPd⁰—NO (1740 cm⁻¹), gas phase or weakly adsorbed NO (1904 and 1837cm⁻¹), NO₂ (1611 cm⁻¹), and nitrate (1583, 1348, and 1300 cm⁻¹) are alsoobserved similar to the corresponding spectra in FIG. 15. Two new weakbands at 1460 and 1680 cm⁻¹ are observed, which are assigned to thesymmetric and asymmetric bending modes of NH₄ ⁺. With increasingtemperature, the bands related to nitrates/nitrite decreasesignificantly, while the bands related to ammonium and ammonia increasefirst at 200° C. and decrease slightly at 240° C. As the temperature isincreased from 150° C. to 240° C., two significant differences areobserved. First, the intensity of the bands increases markedly due toNH₄ ⁺. Second, a new band at 1510 cm⁻¹ is formed due to amide (NH₂).From FIGS. 15 and 16, it can be seen that a significant amount of NH₄ ⁺is formed at temperatures greater than about 200° C. on 1% Pd-5%V₂O₅/TiO₂—Al₂O₃ compared to that on 1% Pd/TiO₂—Al₂O₃.

In order to study the reactivity of NH₄ ⁺ and NH₃ with NO in thepresence of O₂, FTIR spectra (FIG. 17) are obtained over the 1% Pd-5%V₂O₅/TiO₂—Al₂O₃ catalyst at 200° C. following a switch from a feed gascontaining 4000 ppm H₂, 500 ppm NO, and 5% O₂, to one containing 500 ppmNO and 5% O₂. After 1.5 min, the band at 1460 cm¹ due to NH₄ ⁺ almostdisappears (due to reaction with NO+O₂), while the band at 1270 cm⁻¹,due to NH₃, has a slight variation. The result indicates that NH₄ ⁺ ismore reactive than NH₃ in the SCR reaction. After about 10 min, both NH₄⁺ and NH₃ peaks disappear, indicating that both species can react withNO+O₂ at different rates.

Thus, in various embodiments, the 1% Pd-5% V₂O₅/TiO₂/Al₂O₃ catalystoffers significantly high NO conversions using H₂ and CO as a reducingagent in the presence of excess oxygen and at a very high spacevelocity. Compared to others, this catalyst exhibits a higher NOreduction activity as well as a wider operating temperature window.

The description of the invention is merely exemplary in nature and,thus, variations that do not depart from the gist of the invention areintended to be within the scope of the invention. Such variations arenot to be regarded as a departure from the spirit and scope of theinvention.

1. An emissions system for selective catalytic reduction of nitrousoxides (NO_(x)) in lean-burn diesel engine exhaust, the systemcomprising: a catalytic converter having at least one inlet, at leastone outlet, and an interior working environment adapted to receive anddispel an exhaust; a source of a reducing agent comprising H₂ and CO; acatalyst disposed in the interior working environment, the catalystcomprising a compound represented by the formula: X % Pd—Y % V₂O₅/Z,where X is a weight percent between about 0.1 to about 2.0, Y is aweight percent between about 0.1 to about 7.0, and Z is a high surfacearea oxide support material, and a mixing element having a first inputcoupled to the reducing agent source, a second input coupled for receiptof diesel engine exhaust, and an output for injecting the diesel engineexhaust mixed with the reducing agent into the interior workingenvironment of the converter where the catalyst reduces nitrous oxidespresent in the exhaust.
 2. A system according to claim 1, wherein theoxide support material is selected from the group consisting of TiO₂,Al₂O₃, TiO₂—Al₂O₃, Zeolites, Ti-PILC, and combinations thereof.
 3. Asystem according to claim 1, wherein the oxide support materialcomprises TiO₂—Al₂O₃.
 4. A system according to claim 1, wherein theoxide support material is present in an amount having a loading per unitvolume of between about 0.5 to about 10 g/in³.
 5. A system according toclaim 1, wherein the catalyst comprises 1% Pd-5% V₂O₅/TiO₂—Al₂O₃.
 6. Asystem according to claim 1, having a NO_(x) reduction efficiency of atleast about 87%.
 7. A system according to claim 6, having a NOxreduction efficiency of greater than about 95%.
 8. A system according toclaim 1, operating at a temperature between about 125 and about 650° C.9. A system according to claim 1, wherein the reducing agent comprises aratio of H₂:CO of about 3:1.
 10. A system according to claim 1, whereinthe reducing agent further comprises at least one hydrocarbon.
 11. Asystem according to claim 1, further comprising a fuel reformer locatedon-board a vehicle for producing the reducing agent by converting dieselfuel, oxygen, and moisture into H₂ and CO.
 12. A system according toclaim 1, wherein the catalyst further comprises at least one promoterselected from the group consisting of: Ce, Mn, Zr, La, Gd, Nb, Pr, Nd,Sm, Eu, and combinations thereof.
 13. A system according to claim 12,wherein the at least one promoter is present in an amount between about0.01 to about 20% by weight.
 14. A system according to claim 1,operating with the exhaust having a space velocity through the interiorworking environment of between about 9,000 and about 70,000 hr⁻¹.
 15. Asystem according to claim 1, operating with the exhaust having a flowrate of between about 200 and about 700 kg/hr.
 16. A system according toclaim 1, wherein the catalyst comprises a honeycomb structure.
 17. Amethod of selective catalytic reduction of nitrous oxides (NOx) in leanburn diesel engine exhaust, the method comprising: providing a catalyticconverter having at least one inlet, at least one outlet, and aninterior working environment housing a catalyst and configured toreceive and dispel exhaust; mixing a diesel engine exhaust with areducing agent comprising H₂ and CO; and injecting the mixture into theinterior working environment of the converter, wherein nitrous oxidespresent in the exhaust are reduced by the catalyst comprising a compoundrepresented by the formula: X % Pd—Y % V₂O₅/Z, where X is a weightpercent between about 0.1 to about 2.0, Y is a weight percent betweenabout 0.1 to about 7.0, and Z is a high surface area oxide supportmaterial.
 18. A method according to claim 17, wherein the catalystcomprises 1% Pd-5% V₂O₅/TiO₂—Al₂O₃.
 19. A method according to claim 17,wherein the reducing agent comprises H₂ and CO at a ratio of about 3:1.20. A method according to claim 17, wherein the catalyst furthercomprises at least one promoter selected from the group consisting of:Ce, Mn, Zr, La, Gd, Nb, Pr, Nd, Sm, Eu, and combinations thereof.
 21. Amethod according to claim 20, wherein the at least one promoter ispresent in an amount between about 0.01 to about 20% by weight.